Electrical protective apparatus



July 8, 1958 w. L. HURFORD ETAL 2,842,719

ELECTRICAL PROTECTIVE APPARATUS Filed Oct. 18, 1954 f2 ,fa/5r l /0 l L.@Y i erfffy f q i L I /7/ /4 C I Ja l@ 7J i :I

iA y Z0 A P/fazzffm'aca/r 7j myoff/v mmaf frm/v6 Pai/UM am INV N 'TOwma/mim? fawn/M Nia@ HTMKMFY United States Patent ELECTRICAL PROTECTIVEAPPARATUS Winslow L. Harford, Collingswood, and William J. Neely,

Oaklyn, N. J., assignors to Radio Corporation of America, a corporationof Delaware Application Detober 18, 1954, Serial No. 426,956

The terminal lifteen years of the term of the patent to be granted hasbeen disclaimed 11 Claims. (Cl. 317-23) The present invention relates tonew and improved electrical protective apparatus and, more particularly,to apparatus capable of automatically preventing the application ofenergy beyond a predetermined safe value to an electrical load.

While the invention will be described herein as it may be advantageouslyapplied to protecting electron tube laments which are energized from adirect current power supply against over-voltage conditions, it will berecognized that its principles are applicable to other environments inwhich it is necessary or desirable to limit the application of voltageto a load circuit.

It is a primary object of the present invention to provide novel meansfor preventing the application of electrical energy beyond a preselectedvalue to a load.

Another and more specific object of the invention is that of providingnovel circuit means for automatically disconnecting a load from itssource of electrical energy upon the occurrence of an electricalcondition which might render unsafe the continued application of energyfrom source to load.

By reason of the fact that electron tube circuits are rather delicate innature, excessive voltages are capable of causing irreparable and oftencostly damage. There have, therefore, been many proposals in the pastfor electrical protective circuits which operate in a variety of mannersto effect disconnection of a power supply from its load in the event ofequipment failure, over-voltage and the like. Most prior art proposalshave, however, been single-action in nature, that is, capable ofeifecting the necessary de-coupling of power supply and load underunsafe conditions but incapable of recoupling the supply and load uponthe return to safe operating conditions. Where the supply and load arephysically located in proximity to each other, the operator of theequipment may simply effect the recoupling as through the agency of amanual reset switch. When, on the other hand, the supply and its loadare located at a distance from each other or when no operator is inattendance, the problem of returning the equipment to operation is oneof great moment.

Hence it is a further object of the present invention to provide novelprotective circuit means capable of automatically decoupling a powersupply and load under unsafe operating conditions and for automaticallyrecoupling such supply and load upon the return to safe operatingconditions.

In general, the present invention contemplates the realization of theforegoing and other objects through the provision of an electron tubecircuit whose conductivity controls the action of relay means which, inturn, serve to make an electrical connection between a power supply orother energy source and its load. Bias means for said electron tubecircuit determine whether it is to be conductive or non-conductive inaccordance with the condition of the power supply under load. Normally,according to one form of the invention, such means are provided formaintaining the electron tube in a conductive 2,842,719 latented July 8,1958 ICC state so that the relay means are operative to provide acurrent path between supply and load. Upon the occurrence of an unsafecondition (e. g., over-voltage) to which the bias means are responsive,the tube is rendered non-conductive, thereby lactuating the relay meansin a manner to disconnect the load from its supply. A charging circuitin the bias arrangement serves to return the tube to its conductivestate after a short time. delay, thereby permitting the resampling ofthe voltage conditions under load. When such resampling detects that theunsafe condition no longer obtains, the electron tube is permitted toremain conductive, thereby insuring the safe flow of energy betweensupply and load. As will appear more fully hereinafter, the presentinvention is advantageous in that it serves as an automatic recyclingarrangement as set forth brieily above and, moreover, fails safe forlall improper or dangerous operating conditions.

Additional objects and advantages of the present invention will becomeapparent to persons skilled in the art from a study of the followingdetailed description of the accompanying drawing, in which:

Fig. 1 is a schematic circuit diagram illustrative of an embodiment ofthe present invention; and Figs. 2 and 3 are, respectively, equivalentcircuit diagrams of the circuitry of Fig. l for different conditions ofits operation.

Referring to the drawing and, particularly, to Fig. l

thereof, a direct current power supply 10 having positive and negativeoutput terminals l2 and 14, respectively, is adapted to be connectedelectrically to a load circuit 16 in such manner as to energize the loadwith direct current voltage of a predetermined level. The load circuit16 may, by way of illustration, comprise one or more electron tubefilaments and may be located at a substantial distance from the powersupply 10, so that it will be understood that the leads 18 and 20connected respectively to the load terminals 22 and 24 can be in theform of a length of cable, for example, stretched between differenttloors of a building or between any two other remote points. In such anenvironment it will be appreciated that where no operator is inlattendance or where an operator is present only `at the location of theload, any protective circuit which operates to disconnect the load 16from its power supply 10 in the event of over-voltage should also becapable of automatically reconnecting the load and supply once theunsafe condition has been eliminated. The present invention providessuch means in the form of the circuitry contained Within the dotted linerectangle 26. Those elements of the protective circuit 26 which serveactually to Vmake and break the electrical connections between the powersupply' lil andV load 16 are illustrated diagrammatically within therelay box 28. Specifically, .each of the relays 30 and 32 comprises apair of stationary contact members and a movable member. That is, therelay 30 includes 'upper and lower stationary contacts 34 and 36 and amovable member 38 adapted to make connection with either one of thestationary elements. Similarly, the relay 32 includes upper and lowercontact elements 4t) and 42 and a movable member 44. The two movablemembers 38 and 44 of the relays are normally in their uppermost position(i. e. as shown in Fig. l) and remain in that position until theyare'moved downwardly by the magnetic action of a relay coil 46.

The coil 46 is connected'between a source of positive potentialindicated as 2l-B at the terminal 48 and the anode 50 of an electrontube 52 which further includes a control electrode 54, a cathode 56 anda filament 58.

The control electrode 54 of the tube 52-is connected tov the junction 60of resistors 62 and 64 and a capacitor 66, The end of the resistor 62remote-from the junction 60 is connectedto the terminal 48, whilethe endof the resistor 64femote Ifrom that junction is connected via a lead-70and a resistor 72 to the negative voltage terminal 14 of the powersupply 10. The cathode 56 of the control tube is connected via leads 74and 76 to the positive terminal 12 of the supply. The filament winding58 of the tube 52 is connected at one end through a lead 78 to the powersupply terminal 14 and at its other end to one terminal of acurrent-limiting resistor 80 whose other terminal is connected to thejunction 82 between the leads 74 and 76. Finally with regard to thecircuitry associated with the control tube 52, it will be noted that anadditional current-limiting resistor 84 is connected between thejunction 82 and the upper stationary contact 40 of the relay 32.

Insotar as the relays themselves are concerned, it is to be noted thatthe lower contact element 36 of the relay 30 is connected to the lead 20while the movable member 38 is connected directly to the positiveterminal 12 of the power supply. The lower stationary contact 42 of therelay 32 is connected via a lead 86 to the lead 18, while the movablemember 44 is connected directly to the negative terminal 14 of the powersupply.

The operation of the apparatus of Fig. l may, perhaps, be more readilyunderstood from the simplified showings of Figs. 2 and 3. The iirststage of the `operation will be understood as being that in which thepower supply has just `been connected to the circuit 26. Since therelays 30 and 32 are then in their open positions (i. e. as shown inFig. l), the equivalent showing of the apparatus will be as indicated inFig. 2. The source voltage across the terminals 12 and 14 is applied tothe control tube filament 53 through the series current limitingresistor 80 and the filament current is further limited during theWarm-up period of the tube by means of the shunt resistor 84. As thefilament 58 becomes heated, the control electrode 54 of the control tube52 will be brought to a potential which is determined by the division ofvoltage between the resistors 62, 64 and 72, which potential may bechosen to be substantially that of the cathode 56. The capacitor 66will, as a result, become charged to the potential existing at thejunction 60 in the voltage division network. By reason of the describedbiasing of the control electrode 54 with respect to the cathode 56, thetube 52 will conduct heavily through the relay coil 46, thereby movingthe relay members 38 and 42 to their lower positions (hereinafter termedtheir closed positions) with the result that the equivalent circuit willbe as shown in Fig. 3. Closing of the relays 30 and 32 serves to connectthe supply 10 to the load 16 in the following manner (Fig. 1): thepositive terminal 12 of the power supply may be traced through themovable relay member 38 and the stationary contact 36 of the relay 30 tothe lead 20. Similarly, the negative voltage terminal 14 may be tracedthrough the movable terminal 44, stationary contact 46 and lead 86 tothe lead 18. With the relays closed, the shunt resistor 84 in circuitwith the lilament 58 is effectively removed from the circuit, since ithas served its purpose of limiting the filament current initially duringtube warm-up. Of greater signicance, however, is the fact that closingof the relay 32 results in the placing of an etfective short circuitacross the resistor 72, so that the control electrode 54 is at apotential determined by the division of voltage between only theresistors 62 and 64, which potential is less positive than that whichobtained in the condition of Fig. 2. In the condition of the circuitshown in Fig. 3, it will be understood that the capacitor 66 isdischarged to the less positive potential existing at the terminal 60.

By virtue of the fact that, in Fig. 3, the power supply terminals 12 and14 are electively connected to the load 16, the supply voltage at thoseterminals will be lowered from their no-load values. lower potential atwhich the control electrode 54 is returned sets the degree of conductionof the tube 52 at such value as to maintain the current in the relaycoil 46 sufcient to hold the relays in their closed positions andfurther places the operating point of the tube 52 where This fact,coupled with thev it is extremely sensitive to changes in its biaspotentials (viz. as determined by changes in the voltage applied to theload 16).

Assuming next that some portion of the load 16 between the terminals 22and 24 is removed or that an increase inthe supply voltage at the sourceterminals 12 and 14 results in raising the load voltage beyond apredetermined maximum or safe value, the following events will takeplace: the control electrode 54 will be brought to a less positivepotential (i. e. more negative) by reason of the more negative potentialat the terminal 14, and the cathode 56 will be returned to a morepositive potential by reason of a more positive potential at theterminal 12. These two potential changes are suiiicient to bring aboutsuch a bias increase in the tube 52 as to reduce the anode current inthat tube below the value necessary for maintaining the relays in theirclosed position. The movable contacts 38 and 42 of the relays will,therefore, return to their upper positions, thereby returning theprotective circuit to the condition shown by the equivalent diagram ofFig. 2 wherein the voltage supply terminals 12 and 14 are disconnectedfrom the load. In this manner, the protective circuit 26 of the presentinvention serves to prevent the application of an excessive voltagebetween the power supply and its load. In accordance with the invention,however, the c-ircuitry within block 26 is further operative asautomatic resampling means for reconnecting the power supply to its loadonce the unsafe conditions cease to obtain. The manner in which thislatter action is realized will now be described.

As has been stated, the voltage condition which caused the control tube52 to return to its non-conductive state also resulted in the opening ofthe relays 30 and 32, thereby placing the circuit in its condition shownin Fig. 2. The opening of the relays removes the short circuit fromacross the bleeder resistor 72, so that the control electrode 54 of thetube is brought to a more positive potential at a rate determined by thetime constant of the circuit including the capacitor 66 and theresistors 62, 64 and 72 to a potential determined by the value of -i-Band the negative potential at terminal 14 as divided by those resistors.When the capacitor 66 has chargedto its new, more positive potential,the tube 52 will conduct heavily through the relay coil 46, therebyclosing the relays 3Q and 32 once again. Concurrently with the closingof the relay 32, the resistor 72 is again sh-ort-circuited, leaving onlythe resistors 62 and 64 to divide the voltage between the terminals 48and 14. The capacitor 66 then discharges and, assuming that the voltageunder load is within a Safe range (i. e. below the preselected maximumvalue), the rclays 30 and 32 will remain closed, thus permittingcontinued voltage lapplication from the supply to the load. Assuming, onthe other hand, that when the capacitor 66 has discharged the loadvoltage is still outside the safe range, the tube 52 will again be cutoli, causing the relays to open and to disconnect the supply from theload. The circuit 26 will continue to resample the voltage under load inthe manner described until such time as the voltage under load fallsWithin the safe range, whereupon the relays 30 and 32 will remain closedwith the circuit in the condition of Fig. 3.

It will be apparent from an examination of Fig. l that the apparatus ofthe present invention necessarily fails safe (i. e. with the loaddisconnected from the power supply in the event of abnormal operatingconditions other than the above-described over-voltage condition). Forexample, failure of the +B supply connected to the terminal 48 willcause 'the tube 52 to be cut ott, thereby opening the relays. Similarly,failure of the tube 52, as through an open filament or the like, willalso result in the de-energization of the relays.

While the invention has been described in accordance with a specilicembodiment in which a load is supplied with energy from a power supplyhaving terminals which are respectively positive and negative withrespect to ground, it should be understood that the voltage may bepositive only or negative only without appreciably altering theoperation of the apparatus. That is to say, either of the terminals 12and 14 might be at ground potential rather than floating as des'cribedherein.

Further in this vein, it may be noted that if the impedance from thenegative supply terminal 14 to ground is small compared to the value ofthe resistor 72, the return potential for the resistor 64 issubstantially ground potential. In this latter event, the samplingaction or 'recycling which is produced in accordance with the inventionwill be realized as described except for the fact that the charging rateof the capacitor 66 will be changed by reason of the difference in theresistive value of the time constant network. Even where the impedancefrom the terminal 14 to ground is quite low, it will be understood, whenthe load is disconnected through the protective action of the circ-uit,the resistor 72 will be reinserted into the circuit (as shown in Fig. 2)so that the sampling process will be carried out in the manner set forthsupra.

From the foregoing it will be recognized that the present inventionprovides a circuit requiring only a single electron tube associated withrelatively simple and inexpensive components, which circuit servesautomatically to disconnect a load from 4its power supply under unsafeconditions and, moreover, to resample the conditions under loadperiodically whereby to reconnect the load to its supply once the unsafeconditions have been eliminated.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent is:

l. Protective apparatus for automatically disconnect- -ing a source ofelectrical energy from a load under predetermined conditions, whichapparatus comprises: means including switch means connected between suchsource and such load for defining a current path therebetween: electrontube means operatively connected to s'aid switch means for causing saidswitch means selectively to open or complete said current path,depending upon the conductivity of said tube means; means coupled tosaid electron tube and to such source for controlling the conductivityof said tube in accordance with the level of energy supplied by suchsource under load conditions' and in such manner as to change theconduction of said tube in the direction of opening said current pathwhen such energy level departs from a preselected level; and means incircuit with said last-named means for causing the conductivity of saidtube to revert, after a predetermined time interval, to its state inwhich said current path is completed by said switch means, whereby tosample such energy level under load.

2. Protective apparatus for automatically disconnecting a source ofelectrical energy from a load under predetermined conditions, whichapparatus comprises: means including switch means connected between suchsource and such load for defining a current path therebetween: electrontube means operatively connected to said switch means for causing saidswitch means selectively to open or complete said current path,depending upon the conductivity of said tube means; means coupled tosaid electron tube and to such source for controlling the conductivityof said tube in accordance with the level of energy supplied by suchsource under load conditions and in such manner as to change theconduction of said tube in the direction of opening said current pathwhen such energy level departs from a preselected level; and meansoperatively connected to and controlled by said switch means for causingthe conductivity of said tube to revert, after a predetermined timeinterval, to its state in which said current path is completed by saidswitch means, whereby to sample such energy level under load.

3. Protective apparatus for automatically disconnecting a source ofelectrical energy from a load under predetermined conditions, whichapparatus comprises: means including switch means connected between suchsource and such load for dening a current path therebetween: electrontube me-ans operatively connected to said switch means for causing saidswitch means :selectively to open or complete said current path,depending upon the conductivity of said tube means; biasing meanscoupled to said electron tube and to such source for controlling theconductivity of said tube in accordance with the level of energysupplied by such source under load conditions and in such manner as tochange the conduction of said tube in the direction of opening saidcurrent path when such energy level departs from a preselected level;and auxiliary bias means in circuit with said last-named means forperiodically causing the conductivity of said tube to revert to itsstate in which said switch means complete said current path, whereby toresample such energy level under load.

4. Protective apparatus for automatically disconnecting a source ofelectrical energy from a load under predetermined conditions, whichapparatus comprises: means including switch means connected between suchsource and such load for defining a current path therebetween: electrontube means operatively connected to said switch means for causing saidswitch means selectively to open or complete said current path; biasmeans coupled to said electron tube and to such source for controllingthe conductivity of said tube in accordance with the level of energysupplied by such source under load conditions and in such manner as tochange the conduction of said tube in the direction of opening saidcurrent path when such energy level exceeds a preselected level; saidswitch means having means for inserting auxiliary bias means in circuitwith said iirst-named bias means for causing the conductivity of saidtube to revert, after a predetermined time interval, to its state inwhich said current path is completed by said switch means, whereby toresample such energy level under load.

5. Electrical protective apparatus for preventing the application ofenergy from an electrical power supply to a load under predeterminedconditions, which apparatus comprises: switch means having open andclosed positions and connected between such supply and such load in suchmanner as to deline a current path therebetween in said closed position;an electron tube operatively connected to said switch means in suchmanner that the conductivity of said tube determines the opening andclosing of said switch means, said electron tube having aconduction-controlling electrode; circuit means coupled to said powersupply and to said electrode for biasing said tube into that state ofconductivity necessary for closing said switch means, said circuit meansincluding means for varying the biasing of said tube in response to agiven change in such power supply energy so as to cause the conductivityof said tube to change in the direction of opening said switch means;and means controlled by said switch means in said open position forplacing an auxiliary bias on said tube of such Value as to cause theconductivity of said tube to revert to its switchclosing state after apredetermined time interval.

6. Electrical protective apparatus for preventing the application ofenergy from an electrical power supply to a load under predeterminedconditions, which apparatus comprises: switch means having open andclosed positions and connected between such supply and such load in suchmanner as to define a current path therebetween in said closed position;an electron tube operatively connected to said switch means in suchmanner that the conductivity of said tube determines the opening andclosing of said switch means, said electron tube having aconductioncontrolling electrode; circuit means coupled to said powersupply and to said electrode for biasing said tube into that state ofconductivity necessary for closing said switch means, said circuit meansincluding means for varying the biasing of said tube in response to agiven change in such power supply energy so as to cause the conductivityof said tube to change in the direction of opening said switch means;and means controlled by said switch means in said open position foraltering said biasing circuit means for placing an auxiliary bias onsaid tube of such Value as to cause the conductivity of said tube torevert to its switch-closing state after a predetermined time interval.

7. Electrical protective apparatus for preventing the application ofenergy from an electrical power supply to a load under predeterminedconditions, which apparatus comprises: switch means having open andclosed positions and connected between such supply and such load in suchmanner as to define a current path therebetween in said closed position;an electron tube operatively connected to said switch means in suchmanner that the conductivity of said tube determines the opening andclosing of said switch means, said electron tube having a conductioncontrolling electrode; circuit means coupled to said power supply and tosaid electrode for biasing said tube into that state of conductivitynecessary for closing said switch means, said circuit means includingmeans for varying the biasing of said tube in response to a given changein such power supply energy so as to cause the conductivity of said tubeto change in the direction of opening said switch means; and meansincluding a charging circuit controlled by said switch means in saidopen position for placing an auxiliary bias on said tube of such valueas to cause the conductivity of said tube to revert to itsswitch-closing state after a time interval determined by the timeconstant of said charging circuit.

8. Electrical protective apparatus for preventing the application ofenergy from a voltage power supply to a load when the Voltage furnishedby such supply under load departs from a selected level, said apparatuscomprising: switch means having first and second positions and connectedbetween such supply and load so as to define a current path therebetweenin said first position and an open circuit therebetween in said secondposition; switch-operating means including an electron tube having aconduction controlling electrode and coupled to said switch means insuch manner that a certain amount of conduction of said tube isnecessary for maintaining said switch means in said first position;circuit means coupled to said electrode and to such supply fordecreasing the conductivity of said tube below said certain amount inresponse to a departure of said power supply voltage from a selectedlevel, whereby to place said switch means in said second position; andmeans in circuit with said circuit means and controlled by said switchmeans in said second position for increasing the conductivity of saidtube to at least said certain amount, thereby causing said switch meansto revert, after a predetermined time interval, to said first positionwherein the conductivity of said tube is again responsive to the levelof said power supply voltage,

9. Electrical protective apparatus `for preventing the application ofenergy from a voltage power supply to a load when the voltage furnishedby such supply under load departs from a selected level, said apparatuscomprising: switch means having first and second positions and connectedbetween such supply and load so as to define a current path therebetweenin said first position and an open circuit therebetween in said secondposition; switch-operating means including an electron tube having aconduction controlling electrode and coupled to said switch means insuch manner that a certain amount of conduction of said tube isnecessary for maintaining said switch means in said first position;voltage-dividing circuit means having a given divisor and coupled tosaid electrode and such supply for decreasing the conductivity of saidtube below said certain amount in response to a departure of said powersupply voltage from a selected level, whereby to place said switch meansin said second position; and means in circuit with said voltage dividingcircuit means and controlled by said switch means in said secondposition altering said divisor so as to increase the conductivity ofsaid tube to at least said certain amount, thereby causing said switchmeans to revert to said certain amount, thereby causing said switchmeans to revert to said first position wherein the conductivity of saidtube is again responsive to the level of said power supply voltage.

10. Electrical protective apparatus for preventing the application ofenergy from a voltage power supply to a load when the voltage yfurnishedby such supply under load departs from a selected level, said apparatuscomprising: switch means having first and second positions and connectedbetween such supply and load so as to define a current path therebetweenin said first position and an open circuit therebetween in said secondposition; switch-operating means including an electron tube having aconduction controlling electrode and coupled to said switch means insuch manner that a certain amount of conduction of said tube isnecessary for maintaining said switch means in said first position; aresistive voltagedividing network connected between a point of fixedpotential and such source and having an intermediate point connected tosaid electrode such that the conductivity of said tube is decreasedbelow said certain amount in response to a departure of said powersupply from a selected level, whereby to place said switch means in saidsecond position; and means in circuit with and controlled by said switchmeans in said second position for altering the division ratio of saidnetwork in such sense as to increase the conductivity of said tube to atleast said certain amount, thereby causing said switch means to revertto said first position wherein the conductivity of said tube is againresponsive to the level of said power supply voltage.

11. Electrical protective apparatus for preventing the application ofenergy from a voltage power supply to a load when the voltage furnishedby such supply under load departs from a selected level, said apparatuscomprising: switch means having first and second positions and connectedbetween such supply and load so as to define a current path therebetweenin said first position and an open circuit therebetween in said secondposition; switch-operating means including an electron tube having aconduction controlling electrode and coupled to said switch means insuch manner that a certain amount of conduction of said tube isnecessary for maintaining said switch means in said first position;voltage-dividing circuit means including a capacitor coupled to saidelectrode and supply for decreasing the conductivity of said tube belowsaid certain amount in response to a departure of said power supplyvoltage from a selected level, whereby to place said switch means insaid second position; and means operatively connected to and controlledby said switch means in said second position for charging said capacitorto a potential for increasing the conductivity of said tube to at leastsaid certain amount, thereby causing said switch means to revert, aftera time interval determined by the time constant of the charging circuitof said capacitor, to said first position wherein the conductivity ofsaid tube is again responsive to the level of said power supply voltage.

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